Effect of the Internal Tide on Acoustic Transmission Loss at Midfrequencies

Nonlinear internal waves are a common event on the continental shelf. The waves depress the high-gradient region of the thermocline and thicken the surface mixed layer with consequent effect on acoustic propagation. After the waves have passed, it may take several hours for the thermocline to rise to its prewave level. To examine the effect of the rising thermocline, oceanographic and acoustic data collected during the 2006 Shallow Water Experiment (SW06) are analyzed. Midfrequency acoustic data (1.5-10.5 kHz) taken for several hours at both fixed range (550 m) and along a tow track (0.1-8.1 km) are studied. At the fixed range, the rising thermocline is shown to increase acoustic intensity by approximately 5 dB . Along the tow track, the transmission loss changes 2 dB for a source-receiver pair that straddles the thermocline. Using oceanographic moorings up to 2.2 km away from the acoustic receiver, a model for the rising thermocline is developed. This ocean model is used as input to a broadband acoustic model. Results from the combined model are shown to be in good agreement with experimental observation. The effects on acoustic signals are shown to be observable, significant, and predictable.